Aye - Preview is quite a powerful built in AppMac 101: Preview - which can do resize, export formats, crop - all built in. It can even modify PDFs to some extent.

Pretty much all FOSS editors are available in one form or another for Mac - you didn't say what you were coding
Aquamac - Emacs which has a bunch of editing modes which are very powerfulAquamacs: Emacs for Mac OS X